As shown in FIG. 1, an existing passive optical network (PON) system generally includes the following three parts: an optical line terminal (OLT), an optical distribution network (ODN), and an optical network unit (Optical Network Unit, ONU) or an optical network terminal (ONT). In the PON system, transmission in a direction from the OLT to the ONU/ONT is downlink, and transmission in an opposite direction is uplink. Because of properties of light, downlink data is broadcast by the OLT to each ONU. The OLT allocates a transmit timeslot to each ONU to send uplink data. Time division multiplex transmission is used in the uplink direction. The ODN is a passive light splitter, transmits downlink data of the OLT to each ONU, and collects and transmits, to the OLT, uplink data of a plurality of ONUs. The ONU provides a user-side interface for the PON system, and is connected to the ODN in uplink. The ODN usually includes three parts: a passive optical splitter (Splitter), a feeder fiber, and a distribution fiber. Different wavelengths are respectively used in downlink and uplink for a common PON system.
As shown in FIG. 2, a time wavelength division multiplexing passive optical network (TWDM PON) is obtained through extension based on an architecture of a PON. TWDM is an acronym of time division multiplexing (TDM) and wavelength division multiplexing (WDM). The TWDM PON has a same network structure of an entire ODN as the PON, and a main difference lies in that a quantity of uplink and downlink wavelengths increases from one to four or more. In the downlink direction, four transmitters on a side of an OLT respectively emit four different wavelengths, and the four different wavelengths pass through an optical multiplexer, enter a feeder fiber, and then arrive at an ONU. A receiver of the ONU selects only one wavelength for receiving. Therefore, one filter needs to be disposed before the receiver. Four different filters may be prepared for different ONUs because one of the four wavelengths is to be selected. Alternatively, a tunable filter may be used, and configured for different wavelengths according to an actual need, thereby reducing types of filters. In the uplink direction, any ONU emits one of four uplink wavelengths. Therefore, there are four uplink beams at any moment. Similar to the case of filters, the transmitters may be optionally four different lasers, or may be one laser and adjusted to a particular wavelength as required, thereby reducing types of ONUs. After entering the optical distribution network, the four uplink wavelengths arrive at an optical demultiplexer of the OLT. The optical demultiplexer separates the four uplink beams with different wavelengths, which then enter different receivers. WDM on the OLT and the ONU is a filter for aggregating or separating uplink and downlink wavelengths.
Compared with an existing 10G PON system, for a TWDM-PON system, a wavelength multiplexer and demultiplexer need to be additionally introduced on the side of the OLT, to multiplex or demultiplex a plurality of uplink and downlink wavelengths, and a tunable filter needs to be introduced on a side of the ONU, to select a wavelength. A tunable transmitter needs to be introduced for the transmitter. These devices and the like cause an additional insertion loss. In order to be compatible with an existing and deployed optical distribution network ODN network, a higher optical power budget is required between an optical transmitter and an optical transceiver of the TWDM-PON system. To achieve this effect, the following technical solutions are provided in the prior art.
To increase the optical power budget of the system, most commonly seen methods are: (1) increasing an optical power of a transmitter, where a high-power transmitter is used; (2) improving sensitivity of a receiver, where a high-sensitivity receiver is used; (3) adding an optical amplifier.
The method (3) has high costs, and sensitivity of a currently most commonly used receiver, such as an avalanche photodiode (APD), is limited by a current technical level, has been close to a limit, and is difficult to improve in the short term. For a common direct modulated laser, an output optical power of a laser is changed by directly changing a pump current of the laser. Because no external modulator needs to be used, no additional insertion loss is introduced. Therefore, a higher optical power can be transmitted. However, although the direct modulated laser can transmit a higher optical power, a modulation chirp of the direct modulated laser is very large, and signal quality of a transmit signal sharply deteriorates because of dispersion after the transmit signal is transmitted through a fiber, causing a very high optical power penalty (to be specific, causing a significant decrease in receiving sensitivity). This cannot increase the optical power budget.
Based on the foregoing problem, the prior art further provides a high optical power budget solution (where a structure is shown in FIG. 3) of a high-power-based direct modulated laser and electronic dispersion compensation (EDC). The high-power-based direct modulated laser transmits a relatively strong optical power signal, and receiving sensitivity deterioration caused by dispersion is removed by using an electronic dispersion compensation method at a receive end, thereby increasing an overall optical power budget of a link.
The high optical power budget solution of the high-power-based direct modulated laser and the electronic dispersion compensation has the following disadvantages: An electronic dispersion compensation EDC chip is usually very expensive, and there has been no mature solution in the industry, especially for electronic dispersion compensation for a burst signal in uplink. In addition, the electronic dispersion compensation introduces some noise, and is usually more applicable to transmission dispersion compensation of hundreds or thousands of kilometers in the transport network field. For the PON field, there is usually a fiber distance of only 20 kilometers, a residual dispersion amount is still relatively large after the compensation, and therefore, relatively limited sensitivity is improved.